In The Kite Runner, what shocking news does Amir learn when Rahim contacts Amir and tells him to go back to Afghanistan?

It is in Chapter 17 that Amir finds out two pieces of shocking news. Firstly, Rahim Khan tells him about Hassan's death. However, by far the most shocking piece of news concerns the real father of Hassan. Rahim Khan tells Amir that Ali, the man Amir had always assumed to be Hassan's father, was actually sterile, indicating that someone else was responsible for the birth of Hassan. Amir is cleary shocked by this news that...

It is in Chapter 17 that Amir finds out two pieces of shocking news. Firstly, Rahim Khan tells him about Hassan's death. However, by far the most shocking piece of news concerns the real father of Hassan. Rahim Khan tells Amir that Ali, the man Amir had always assumed to be Hassan's father, was actually sterile, indicating that someone else was responsible for the birth of Hassan. Amir is cleary shocked by this news that Hassan was actually his half-brother:

I felt like a man sliding down a steep cliff, clutching at shrubs and tangles of brambles and coming up empty-handed. The room was swooping up and down, swaying side to side.

Of course, this news makes Amir's betrayal of Hassan when they were children even worse, as what he did he did to his brother. Notice how Rahim Khan uses this to force Amir to go to Afghanistan to rescue Sohrab - Amir cannot ignore the plea for help to rescue his nephew, especially when he did so little to help Sohrab's father in a similar situation.